The present invention is related to solar cells.
The present invention is also related to a cost-effective process of realization of solar cells.
Most solar cells described in the prior art can be subdivided into several categories according to their general structure.
One of these categories is the group of the so-called back-contacted solar cells, meaning that both ohmic contacts to the two oppositely doped regions of the solar cells are placed on the back or non-illuminated surface of the solar cell. This concept avoids shadowing losses caused by the front metal contact grid on standard solar cells.
The most straightforward way to fabricate back contact solar cells is to place the carrier collecting junction between semiconductor regions of opposite doping close to the back surface of the cell (xe2x80x9cback-junctionxe2x80x9d cell). The document xe2x80x9c27.5-Percent Silicon Concentrator Solar Cellsxe2x80x9d (R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Letters, Vol. ED-7. No. 10, October 1986) describes such a device.
Since the majority of photons are always absorbed close to the front surface of the cell, the generated carriers in these regions have to diffuse through the entire base region of the cell towards the carrier collecting junction close to the back surface. For this concept, high quality material with minority carrier diffusion lengths longer than the cell thickness is needed, which makes this solution not applicable for most solar grade materials which generally have short diffusion lengths. Additionally, a perfect front surface passivation is required for cells having the carrier collecting junction close to the back surface.
The largest group of solar cells has the carrier collecting junction close to its front surface. The current from these solar cells is collected by a metal contact to the doped region on the front surface and by a second contact to the oppositely doped region on the back surface. Although this front grid structure can be optimized relatively easily in order to get high collection efficiencies, the trade off between resistance losses and shading losses necessitates a coverage of the front surface by 10-15% of the total area.
Another group of solar cells combines the two approaches. Such solar cells have both external contacts to the oppositely doped regions on the back surface and the collecting junction close to the front surface. The collected current from the front surface is lead through openings, which extend through the entire wafer, to the back surface. Using this structure, shading losses normally arising from the front metallization grid are greatly reduced.
Several patents make use of this approach.
Documents U.S. Pat. Nos. 4,227,942 and 4,427,839 disclose solar cell structures in which the metal contacts to both oppositely doped regions are placed on the back surface of the device. The connection to the front carrier collecting junction is realized by chemically etched vias which are arranged in an array. The metal grids and chemical etch mask are defined by photolithography. Photolithography, however, is an expensive processing step and difficult to implement into industrial solar cells production.
The document U.S. Pat. No. 4,838,952 discloses a similar structure wherein an array of holes is created with photolithographically defined areas using chemical etching. In this case, the holes do not extend from the top surface to the back surface of the device. They only extend from the back surface to the junction region. Due to the lower doping density at the junction region compared to the surface where the contacts are normally placed, the contact resistance is expected to be higher with this device if industrial metallization techniques such as screen printing are used. The disadvantages of photolithography also apply to this method.
The document U.S. Pat. No. 3,903,427 also describes a solar cell with an array of holes, machined by mechanical, electron beam or laser drilling, in order to lead the collected current from the front surface of the solar cell to the back surface. In this case, the metal contacts to the regions of opposite polarity are placed on the back surface, one above the other separated by a dielectric layer. This device makes it also necessary to have an insulating dielectric layer along the walls of the holes. This layer is difficult to combine with industrial metallization techniques such as screen printing metal paste and firing the metal paste which dissolves dielectric layers.
The document U.S. Pat. No. 4,626,613 discloses solar cells with both contacts on the back surface and an array of holes connecting front and back surface. The holes are used for conducting current from the front surface to the metal grid on the back surface of the appropriate polarity. The holes are machined by laser drilling or by scribing a set of parallel spaced grooves on the front and rear surface. The two sets of grooves on both surfaces are oriented perpendicularly so that after an appropriate etching process, holes are revealed at the points of intersection.
A similar structure is shown in U.S. Pat. No. 5,468,652, wherein the cell structure uses an array of laser drilled holes to conduct current collected on the front surface to the back surface where the metal contacts to the oppositely doped regions are placed. Although this latter case offers also some simplifications to cell processing compared to the ones suggested hereabove, there are still some common drawbacks of cell structures which make use of a large number of holes for electrically connecting the two surfaces of a cell.
In order to avoid resistive losses caused by current crowding effects within the heavier doped surface layer of the cell around the holes, the holes need to be spaced 1-1.5 mm to each other in both dimensions. On large area solar cells (10xc3x9710 cm2) a total number of more than 10000 holes would be necessary. Other difficulties arise from the metallization point of view. The close spacing of holes demands very narrow alignment tolerances for the two metal grids on the back surface. The large number of holes associated with the structures disclosed in the patents listed above makes these cell structures expensive and not well suited for mass production.
The document U.S. Pat No. 3,903,428 discloses a solar cell structure that uses one centrally located hole in combination with a metal grid on the cell""s front surface to lead the collected current from the front to the back surface. The disclosed structure is best suited for round devices of small area due to increased resistive losses caused by current crowding round the centrally located hole. U.S. Pat. No. 3,903,428 also does not allow a second collecting junction to be placed on the back surface of the cells which would be possible with some of the structures discussed above.
The document JP-63-211773-A describes a solar cell structure where removing the external contact from the front surface increases the active area and makes both polarity contacts accessible from the back surface. Incident light generates electron-hole pairs in the bulk of the structure. Excess minority carriers diffuse towards the collecting junction formed by epitaxial growth at the front surface. Once they crossed the junction they can diffuse as majority carriers towards a metal contact, which is part of a conduction path towards external contacts at the back surface of the cell. The conduction path between the front and back surface is foreseen through a limited number of holes. The diffusion of the minority carriers through the whole wafer makes this approach difficult to use for lower quality materials. The distance a minority carrier can diffuse through the bulk region before it recombines is limited by the material quality. For high quality material, minority carriers can travel several times the base width before recombining. However, the diffusion length in low-grade material can be lower than the cell structure. In this case, carriers generated deep within the structure have a small chance to reach the front surface where they can be collected.
The present invention aims to suggest the realization of a solar cell structure suitable for solar grade materials that overcomes the limitations of the above mentioned structures of the state of the art while maintaining industrial applicability.
The present invention aims to suggest a cost-effective process of realization of solar cells.
The first object of the present invention is to suggest a solar cell in a semiconductor substrate comprising at least a radiation receiving front surface and a second surface, said substrate comprising a first region of one type conductivity, and a second region of the opposite conductivity type with at least a first part located adjacent to the front surface and with at least a second part located adjacent to the second surface, said front surface having conductive contacts to said second region and said second surface having separated contacts to said first region and to said second region, wherein the contacts to said second region at the second surface are connected to the contacts at the front surface through a limited number of vias.
With a limited number of vias, it is meant a number of vias that can be fabricated in an industrially feasible (short enough) time frame. Thus, the number of vias is about 100 or lower for a 10xc3x9710 cm2 solar cell, for instance of the order of 10-20.
If said second surface is the back surface of the substrate, separated contacts to said first region and said second region are created on the back surface.
The second region is defined by the doped region of the substrate and can be either of n-type or p-type while the first region is then of p-type or n-type.
Preferably, the vias extend from the front surface to the back surface of the substrate and are cone shaped or cylindrical shaped.
The conductive contacts on the front surface are formed by a number of narrow metal lines, each part of a conductive path towards at least one via opening on the front surface.
Both contacts at the back surface serve as external contacts for the device. The contact to the second region at the back surface serves to pick up and transport carriers collected at the front junction between the regions by means of the connection to the contact at the front surface through the vias, and additionally to pick up and transport the carriers collected at the junction close to the back surface.
A second object of the present invention is to suggest a process of realization of a solar cell consisting essentially in a semiconductor substrate having a first region of one conductivity type and a second region of the opposite conductivity type, said substrate being defined by a front surface intended to receive the radiation and a second surface intended to receive contacts to the first region and the second region, said process comprising at least the following steps:
machining a number of vias through the substrate;
chemically etching said vias;
introducing phosphorous or any other dopand in the substrate including the walls of the vias in order to create a second region;
forming contacts to both first region and second region of the solar cell, said contacts comprising at least contacts to the second region on the front surface and external contacts being on the second surface;
metallizing the vias in a way that the metallization forms a conduction path between the contacts to the second region on the front surface and at least one of the external contacts on the second surface.
Preferably, the steps of forming the contacts and metallizing the vias can be performed substantially simultaneously.
The introduction of a dopand into the substrate can be done e.g. by diffusion, ion implantation of a dopand into the substrate.
The formation of metal contacts can be for example by screen printing and firing, evaporation and/or any other technique of deposition of metal.